Preliminary studies have shown that Ca ions can reverse the myocardial depression caused by anesthetic agents (halothane and nitrous oxide) in cat papillary muscle. The phasic contraction produced by a 3msec electrical pulse is more severely depressed by halothane than is the tonic contraction produced by tetanizing the muscle. This suggests actions both at the cell surface and in the contractile proteins. More direct evidence for actions of halothane and nitrous oxide on the contractile proteins was obtained by showing anesthetic inhibition of Ca ion sensitive ATPase and suppression of ATP induced myofibrillar contraction. Reconstitution studies suggest that the depression results from an interaction with the troponin-tropomyosin complex. We plan to determine the molecular mechanism and the site of action of several anesthetics on the troponin-tropomyosin complex. In addition, we plan to measure rapid Ca uptake and release by the sarcoplasmic reticulum as affected by anesthetics using the murexide method developed by one of the investigators (T. O.).